This invention relates to method and apparatus for the determination of the amount of a particular type of volatile electrolyte present in a stream. It includes a continuous flow system in which a volatile component that will dissolve in water or other solvent to form an electrically conducting solution is transferred from a sample stream to a deionized water (or other) stream by diffusion through a gas permeable membrane or air gap. The water stream with the dissolved component passes through an electrical conductivity cell for quantification of the concentration of the volatile component in the original sample.
The invention may, for example, be used for Kjeldahl nitrogen, i.e., ammonium nitrogen, determination. It may also be used for determination of dissolved carbon dioxide. It can be used to determine any constituent that will diffuse through a membrane (or air gap) as a gas and that will dissolve in a sample receiving stream to produce a solution that will conduct electricity. Tests have shown its usefulness in detecting methylamine and dimethylamine when the sample stream is mixed with alkali, and acetate when the sample stream is mixed with acid. The invention can be used to determine nitrate nitrogen and ammonium by inserting a small column of Devarda alloy in the sample stream to reduce the nitrate to ammonium.
The invention also responds to atmospheric carbon dioxide levels when air is pumped through the sample and reagent channels, in place of solutions. Thus, the invention can be used as a gas analyzer. It can be made selective as a gas analyzer, at least for some constituents, by selectively absorbing unwanted constituents that produce a response from the gas sample stream.
Heretofore, the leading systems for detection of volatile components in liquids have involved either colorimetry or gas-sensing electrodes. For example, one available instrument can be set up with either one of these two detection systems. In testing for the ammonium ion by the colorimetry system, the sample stream is mixed with reagents that react with ammonium to produce a new, colored, compound that absorbs light. This combined stream then passes through a cell where the amount of light that is absorbed by the stream is measured. The light absorbed is then related to the concentration of ammonium in the original sample stream. In such a system, the original sample must be free of suspended material (turbidity) or colored substances, as they would also absorb light and lead to error. If the sample is not free of turbidity and color, it must be pretreated to remove these before processing on the instrument.
In its other mode, this prior-art instrument uses a gas-sensing ammonia electrode. Here the sample stream is mixed with a reagent and passed through a flow-through fitting attached to the electrode. The electrode consists of a pH-sensitive surface coated with a thin film of solution and separated from the sample by a gas permeable membrane. Ammonia from the sample diffuses through the membrane and dissolves in the thin film of solution causing a change of pH. This pH change is related to the ammonium concentration in the original sample. While this system will tolerate color and turbidity, it has the disadvantage of not being very stable; that is, the detection system tends to drift and so it must be recalibrated frequently. These gas-sensing ammonia electrodes also require frequent servicing; they must be disassembled, cleaned, the membrane replaced, and then the electrode reassembled.
Another major disadvantage is that this prior-art system cannot tolerate samples with even moderate concentrations of dissolved substances (salts, acids, sugars, etc.). Samples with high concentration of dissolved substances must be diluted before processing. If a sample with considerable dissolved substances is run, the results are in error, and the ammonia electrode exhibits a memory effect; several samples following the one with considerable dissolved substances are also in error.
Various procedures for the determination of ammonia by electrical conductivity measurements have been reported. In 1942 Hendricks et al in 5 Ind. Eng. Chem. Anal. Ed. 23-26 described a vacuum distillation procedure in which the ammonia was collected in boric or sulfuric acid and determined by the change in conductivity of the acid. In a similar procedure, Appleton 42 Chemist-Analyst 4-7 (1953), distilled ammonia into boric acid, diluted the solution to known volume, and determined ammonia from conductivity changes. Shaw and Staddon in 35 Jour. Exper. Biol. 85-95 (1958) used a diffusion cell to transfer ammonia from the sample to sulfuric acid for its subsequent determination by conductivity. Friedl in 48 Anal. Biochem. 300-306 (1972) determined ammonia from the rate of change of conductivity of a small volume of sulfuric acid as it absorbed ammonia from a sample in a diffusion cell. Separation of ammonia by distillation (See Keay et al 94 Analyst 895-899 (London, 1969) and dialysis (Technicon Corporation, Tarrytown, N.Y., Industrial Method No. 330-74A/A) have been employed in automated colorimetric methods.
Diffusion through plastic tubing has been reported as a technique for separating gaseous components of samples. Kollig et al, 9 Environ. Sci. Technol. 957-960 (1975) used silicone rubber tubing in a device for sampling waters for dissolved oxygen, nitrogen and carbon dioxide determinations. Scarano and Calcagno, 47 Anal. Chem. 1055-1065 (1975) determined dissolved carbon dioxide from pH changes in a bicarbonate solution flowing through a Teflon tube immersed in the sample. Westover et al, 46 Anal. Chem. 568-571 (1974) described a sampling device for mass spectrometry that was based on gas permeation through silicone rubber hollow fibers.